Each of semiconductors such as silicon or gallium arsenic is constituted of a single crystal and is used for memories and the like of small to large computers, and an increase in capacity, a reduction in cost, and improvement in quality of memory devices have been demanded.
As one of single-crystal pulling methods for manufacturing single crystals which meet these demands of the semiconductors, there has been conventionally known a method for manufacturing a semiconductor having a large diameter and high quality by applying a magnetic field to a molten semiconductor material contained in a crucible and thereby inhibiting heat convection produced in a melt (which is generally referred to as a magnetic Czochralski (MCZ) method).
An example of a single-crystal pulling apparatus using the conventional CZ method will now be described with reference to FIG. 10. A single-crystal pulling apparatus 100 in FIG. 10 includes a pulling furnace 101 having an openable/closeable upper surface, and includes a crucible 102 in this pulling furnace 101. Further, a heater 103 for heating and melting a semiconductor material in the crucible 102 is provided around the crucible 102 in the pulling furnace 101, and a superconducting magnet 130 having a pair of superconducting coils 104 (104a and 104b) incorporated in a refrigerant container 105 as a cylindrical container (which will be referred to as a cylindrical refrigerant container hereinafter) is arranged on an outer side of the pulling furnace 101.
At the time of manufacturing single crystals, a semiconductor material 106 is put in the crucible 102 and heated by the heater 103, and the semiconductor material 106 is molten. A non-illustrated seed crystal is moved down and inserted into this melt from above, e.g., a central portion of the crucible 102, and the seed crystal is pulled in a pulling direction 108 at a predetermined velocity by a non-illustrated pulling mechanism. Consequently, a crystal grows in a solid and liquid boundary layer, and a single crystal is generated. At this time, when fluid motion of the melt induced by heating of the heater 103, i.e., the heat convection is produced, a dislocation of the single crystal to be pulled is apt to occur, and a yield rate of single-crystal production is lowered.
Thus, as a countermeasure, the superconducting coils 104 of the superconducting magnet 130 are used. That is, the semiconductor material 106 which is the melt undergoes motion suppressing power by lines of magnetic force 107 produced by energization to the superconducting coils 104, the growing single crystal is slowly pulled upward with pulling of the seed crystal without producing the convection in the crucible 102, and the single crystal is manufactured as a solid single crystal 109. It is to be noted that, although not shown, the pulling mechanism for pulling the single crystal 109 along a crucible central axis 110 is provided above the pulling furnace 101.
Next, an example of the superconducting magnet 130 used in the single-crystal pulling apparatus 100 shown in FIG. 10 will now be described with reference to FIG. 11. This superconducting magnet 130 has the superconducting coils 104 (104a and 104b) contained in a cylindrical vacuum container 119 through the cylindrical refrigerant container. In this superconducting magnet 130, the pair of superconducting coils 104a and 104b facing each other through a central portion of the vacuum container 119 are accommodated. The pair of superconducting coils 104a and 104b are Helmholtz-type magnetic coils which generate magnetic fields parallel to the same lateral direction, and, as shown in FIG. 10, the axisymmetric lines of magnetic force 107 are generated to the central axis 110 of the pulling furnace 101 and the vacuum container 119 (a position of this central axis 110 is referred to as a magnetic field center).
It is to be noted that, as shown in FIGS. 10 and 11, this superconducting magnet 130 includes a current lead 111 through which a current is introduced to the two superconducting coils 104a and 104b, a small helium refrigerator 112 for cooling a first radiation shield 117 and a second radiation shield 118 contained in the cylindrical refrigerant container 105, a gas discharge pipe 113 through which a helium gas in the cylindrical refrigerant container 105 is discharged, a service port 114 having a replenishing port from which a liquid helium is replenished, and others. The pulling furnace shown in FIG. 10 is arranged in a bore 115 of such a superconducting magnet 130.
FIG. 12 shows a magnetic field distribution of the above-described conventional superconducting magnet 130. As shown in FIG. 11, in the conventional superconducting magnet 130, since the pair of superconducting coils 104a and 104b facing each other are arranged, a magnetic field gradually increases toward both sides in each coil arranging direction (an X direction in FIG. 12), and the magnetic field gradually decreases toward an up-and-down direction in a direction orthogonal to the former (a Y direction in FIG. 12). In such a conventional configuration, since a magnetic field gradient in the range of the bore 115 is too large as shown in FIG. 12, suppression of the heat convection produced in the molten single crystal material is unbalanced, and magnetic field efficiency is poor. That is, as indicated by hatched lines representing a region having the same magnetic flux density in FIG. 12, magnetic field uniformity is not good in a region near a central magnetic field and its vicinity (i.e., a cross shape which is elongate from right to left and up and down is formed in FIG. 12), and hence there arises a problem that a heat convection suppressing effect is low and a high-quality single crystal cannot be pulled.
To solve the problem, as shown in FIG. 13(a) and FIG. 13(b), Patent Document 1 discloses that the number of superconducting coils 104 is four or more (e.g., 104a, 104b, 104c, and 104d), these coils are arranged on planes in a cylindrical container concentrically provided around a pulling furnace, the respective arranged superconducting coils are set to directions in which they face each other through an axial center of the cylindrical container, and an arrangement angle θ (see FIG. 13(b)) at which each pair of superconducting coils adjacent to each other face the inner side of the cylindrical container is set to a range of 100 degrees to 130 degrees (i.e., a center angle α (see FIG. 13(b)) between coil axes of adjacent to each other with the X axis at the center is 50 degrees to 80 degrees). Consequently, a lateral magnetic field which has a reduced magnetic field gradient and excellent uniformity can be generated in a bore 115, a magnetic field distribution having a concentric shape or a square shape can be produced on a plane, unbalanced electromagnetic force can be greatly suppressed, thus a uniform magnetic field region in the pulling direction can be improved, a magnetic field in the lateral magnetic field direction becomes substantially horizontal, manufacture of a high-quality single crystal can be realized by suppression of the unbalanced electromagnetic force, and this patent Document also discloses that a high-quality single crystal can be pulled with a good yield rate by this single-crystal pulling method.
That is, in each magnetic field distribution shown in FIG. 14 to FIG. 18 showing the magnetic field distributions when the arrangement angle θ of the superconducting coils 104a, 104b, 104c, and 104d is set to 100 degrees, 110 degrees, 115 degrees, 120 degrees, and 130 degrees (i.e., the center angle α between the coil axes is 80 degrees, 70 degrees, 65 degrees, 60 degrees, and 50 degrees), a central magnetic field is uniformly arranged in a sufficiently large region. On the other hand, a width of the central magnetic field in the Y direction is extremely narrow when the arrangement angle θ is as small as 90 degrees (the center angle α between the coil axes is 90 degrees) as shown in FIG. 19, and the width of the central magnetic field in the X direction is extremely narrow when the arrangement angle θ is as large as 140 degrees (the center angle α between the coil axes is 40 degrees) as shown in FIG. 20.
Thus, in the superconducting magnet 130 in FIG. 13, when the arrangement angle θ is set in the range of 100 degrees to 130 degrees, a uniformly distributed magnetic field having a concentric shape or a square shape can be provided in the bore 115.